Very low frequency subminiature active antenna

ABSTRACT

A subminiature active antenna includes two non-magnetic conductive plane elements in spaced parallel relationship separated by a distance of the order of the principal dimension of the plane elements. A conductive member connected to a first plane element, extends at right angles through the second plane element, and is connected to a capacitance multiplier circuit grounded to the second plane member. The capacitance multiplier circuit has an input impedance of phase and amplitude substantially matching the conjugate aggregate impedance presented to it by the antenna members which are thus caused to appear to a received wave field as a much larger capacitance and consequently develop a greater useful signal.

United States Patent 1 Firman 3,714,659 Jan. 30, 1973 VERY LOW FREQUENCY SUBMINIATURE ACTIVE ANTENNA Inventor: Carl M. Firman, 6222 Madeline St.,

San Diego, Calif. 92115 Filed: Dec. 10, 1968 Appl. No.: 782,591

U.S. Cl. ..343/70l, 343/828, 343/848, 325/374, 325/318 Int. Cl..... .,....II0lq 1/26, I-IOlq 1/48 Field of Search ..343/701, 848; 325/365-366, 373-376, 318

References Cited UNITED STATES PATENTS 11/1930 Bethenod ..343/848 X 5/1968 Copeland et al ..343/701 X OTHER PUBLICATIONS And Now the Mini-Antenna in Time May 19, 1967 page 124 Tiny Antennas Push Stateof-Art Mayes in Electronics World March 1968 page 4952 Primary Examiner-Eli Lieberman Assistant Examiner-Marvin Nussbaum Attorney-Joseph C. War field, Jr., George J. Rubens and John W. McLaren [57] ABSTRACT A subminiature active antenna includes two non-magnetic conductive plane elements in spaced parallel relationship separated by a distance of the order of the principal dimension of the plane elements. A conductive member connected to a first plane element, extends at right angles through .the second plane element, and is connected to a capacitance multiplier circuit grounded to the second plane member. The capacitance multiplier circuit has an input impedance of phase and amplitude substantially matching the conjugate aggregate impedance presented to it by the antenna members which are thus caused to appear to a received wave field as a much larger capacitance and consequently develop a greater usefuksignal.

7 Claims, 3 Drawing Figures 22v Dc /6 2 l fgb'""' f I 1 l l l ISM: l7 l8 9 F i l23 l H J! l\ 1 22 g l I- .1

PATENTEllJAuso ms 3,714,659 SHEET 2 BF 2 1 NVENTOR.

A TTOR/VEYS 5 a L M. F/RMA/V VERY LOW FREQUENCY SUBMINIATURE ACTIVE ANTENNA STATEMENT OF GOVERNMENT INTEREST The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION The present invention is particularly desirable in the lower frequency ranges including LF, VLF, and ELF. For purposes of explanation, however, it may be desirable to relate the advantages and features of the present invention to more conventional receiving antennas as employed in the VLF range. The more conventional VLF receiving antennas currently in use fall into two general classes. The first class includes E field sensors which are usually monopole antennas, while the second class includes H field sensors which are usually air core or magnetic core loop antennas. Inasmuch as the subminiature active antenna of the present invention is essentially an E field sensor, its comparison will be limited to E field sensors and the H field sensors will not be discussed.

The more conventional E field sensors have the disadvantages of large physical size and the requirement of a large ground plane. These requisites, as a consequence, demand a large physical space for installation of the E field conventional sensor. For example, within the VLF range, a typical E field sensor may be of the order of 35 feet tall.

Another more modern type of E field sensor is known as the probe type which senses the energy such as may be developed in the VLF range on metal structures which typically, in a naval installation, may comprise the mast of a ship. The probe type of conventional antenna is remotely tuned to the same frequency as the receiver, therefore it presents an additional band limiting element in the antenna receiver link. Accordingly, if it is desired to make VLF transmissions having wider bandwidths than are available with the probe type antennas, the use of probe type antenna can impose a severe and undesirable restriction.

SUMMARY OF THE INVENTION The present invention comprises two relatively small plane elements which are positioned in spaced, parallel relationship with respect to each other and having a conductive element which may take the form of a monopole electrically connected ,to the approximate center of one of the plane elements. The conductor member, in the form of a monopole, extends at right angles from the plane member to which it is electrically connected, passing through an aperture or opening in the second plane member which, it will be recalled, is disposed in spaced, parallel relationship with respect to the firstplane member.

The conductive member is electrically connected t provide an input signal to a capacitance multiplier circuit located adjacent to the second plane member and grounded to it. The capacitance multiplier circuit employed in the present invention is characterized by having an input impedance of phase and amplitudesubstantially matching the conjugate aggregate impedance presented to it by the combination of the described antenna members including the first non-magnetic conductive plane member, the conductive member which may take the form of a monopole connected to the center of the first plane member and the second nonmagnetic conductive plane member having an aperture or opening through which the conductive member in the form of a monopole passes for electrical connection to provide an input to the capacitance multiplier circuit. In a typical preferred embodiment the subminiature active antenna of the present invention operative in the VLF range may comprise first and second plane members each approximately 12 inches square and separated from each other in parallel disposition by a distance of approximately 12 inches. Thus, the entire antenna including a transistorized solid state capacitance multiplier circuit may be contained within the spatial volume of little more than 1 cubic foot.

Those skilled in the antenna arts will readily appreciate that the active portion of the antenna, i.e., the capacitance multiplier circuit, may be installed at the bottom of the antenna and it need not be integrated with the physical structure of the antenna as was requiredwith some prior art developments such as those of Meinke, for example. Moreover, the concept of the present invention includes an antenna which provides a self-contained ground plane, uses an active circuit instead of a passive circuit, and canbe employed to replace conventional receiver RF amplifier-antenna systems which could significantly and desirably reduce the number of component parts required.

Accordingly, it is a primary object of the present invention to provide a broadband antenna which is not limited in the information bandwidth of the received signal.

Another important object of the present invention is to provide an antenna of relatively very small physical size in which optimum efficiency is achieved and intermodulation and mixing are reduced to tolerable levels.

A further, most important object of the present invention is to provide a small antenna which includes a self-contained ground plane.

A further object of the present invention is to provide such an antenna which requires no additional tuning.

Yet a further object of the present invention is to provide such an antenna which is readily adapted to be constructed in a physically rugged manner.

Another object of the present invention is to devise an antenna which is readily adaptable to be constructed of a variety of materials.

An additional object of the present invention is to provide a high performance antenna which can be fabricated as a permanently-sealed unit.

These and other advantages, features, and objects of .the present invention will be better appreciated from an understanding of the operation of a preferred embodiment when taken together with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram, partially in perspective view illustrating a preferred embodiment of the present invention;

FIG. 2 is a perspective schematic drawing illustrating a variant form of the present invention; and

FIG. 3 is a perspective drawing illustrating an actual form of VLF antenna embodying the teaching and concept of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 illustrates a first non-magnetic conductive plane member which is disposed in a spaced, parallel relationship relative to a second non-magnetic conductive plane member 11. A conductive member 12, which may take the form of a monopole, is electrically connected to the plane member 11 at its approximate center as indicated at 13 and extends therefrom in a right angular disposition through an aperture 14 in the second plane member 1 1.

The conductor or monopole 12 is connected as the input signal to a capacitance multiplier circuit as represented by the schematic arrangement illustrated within the dash lines. The capacitance multiplier circuit 15 is contained within a shielded enclosure, as represented by the dash lines, and is grounded to the second plane member 11 so that the latter functions in the manner of a ground plane. The vertical monopole member 12 is connected through a coupling capacitor 16 to the base of a transistor 17. The circuit 15 functions in the manner of a capacitance multiplier'to significantly increase the apparent capacitance value presented by the two plane members 10 and 11. The circuit 15 resembles a three-stage Darlington emitterfollower but there are several very important aspects in which the circuit 15 differs from the more conventional emitter-follower.

For example, the transistors 17, 18, and 19 must be carefully selected to present an apparent inductive input impedance. The phase and amplitude of the impedance presented by the transistors must match the conjugate impedance of the antenna elements 10, 11, and 12 in a particular manner. The magnitude of the aggregate input impedance presented by transistors 17, 18, and 19 must be matched as closely as possible over the bandwidth of interest to the impedance magnitude of the antenna elements 10, 11, and 12. The phase angle of the input impedance presented by transistors 17, 18, and 19 in the aggregate must be of the same angle as that presented by the antenna elements 10, 11, and 12 but of an opposite sign, i.e., essentially inductive in character. The necessary measurements may be made with the transistors energized in the circuit and across the bandwidth of interest as ascertained from a conventional vector impedance meter. This type of meter may be connected to the emitter-base junction and, if desired, the same impedance vector meter may be used to measure the concomitant antenna characteristics.

The capacitance presented by the two plane members 10 and 11 together with the monopole element 12 is inserted across the base-emitter junctions of transistors 17, 18, and 19 so that the circuit looks like a single transistor to the antenna. The multiple compound Darlington circuit employing three transistors is used to achieve the desired aggregate electrical characteristics. The circuit 15 is biased at a low level by means of a l megohm resistor 20 which is connected. from the collector of transistor 19 to the base of transistor 17 and also to an appropriate source of DC potential as indicated at terminal 21 to be 21 +22 DC voltage.

It should be noted that there are no circuit elements other than the antenna elements 10, 11, and 12 connected from the base of the transistor 17 to ground. This allows the antenna elements 10, 11, and 12 to look directly into the circuit as represented by the base-toemitter junction of the transistors 17, 18, and 19, plus the addition of the resistive value of 180 ohms of the resistor 22. Accordingly, the circuit 15 operates to cause a relatively small'capacitive value of the antenna elements 10, 1 1 and 12 to appear as a much larger capacitive value to a received field. These characteristics of operation allow the subminiature active antenna of the present invention to function as an efficient sensing device for electromagnetic radiation within its designed bandwidth.

For example, an embodiment of the present invention substantially is represented in FIG. I and employing a 12 inch square configuration of plane members separated by a distance of approximately 12 inches and using transistors of the 2N5l82 type has operated in the VLF region very efficiently. In actual practice, the embodiment of the present invention as represented in FIG. 1 was coupled as the RF input of a VLF receiver having a 50 to ohm input impedance. The circuit was then energized from an appropriate DC voltage source and produced its output at the output terminal 23 as shown in FIG. 1. In actual use, it was found best to locate the entire antenna assembly 1 or 2 feet above the surface of the earth. Further, it was discovered that the length of coaxial cable which may be connected from the output terminal 23 of the antenna may vary considerably without affecting the performance of the antenna. Cable links as short as 2 feet were employed and varied to lengths as long as 40 feet with the indication being that the antenna would function normally with even longer lengths of coaxial cable to transmit the output of the antenna as generated at the output terminal 23.

The antenna of the present invention has the advantage of being readily adaptable to constructions of a variety of materials such as plastic, fiberglass, bakelite, vinyl or any other non-conductive material, with the upper and lower ground plane portions sprayed or painted with conductive paint or plated directly onto a. basic plastic framework. Moreover, the antenna of the present invention, being containable in its VLF version within little more than 1 cubic foot volume, is readily adaptable to be made as a permanently sealed unit.

The VLF version of the embodiment of the present invention as represented in FIG. 1 operated most efficiently within a designed bandwidth range of l4kHz to SOkI-lz. The same VLF version of the antenna'of the present invention exhibited a gain of from 20 to 40db better than a long wire type antenna of approximately one hundred feet in length, thus demonstrating its efficiency and desirability over more conventional forms of antennae. This advantage not only includes the attractive minimal size in which the antenna of the present invention may be embodied, but also its ability to receive signals which more conventional antennae could not receive in a clear and intelligible form.

The present invention embodied in a form operative in the VLF range had a dynamic range in excess of 60db and noise levels below 3 microvolts -for lkI-lz bandwidth. Some minor intermodulation and mixing byproducts were noted in experimentation between approximately 300kI-Iz and 535kHz. These byproducts are believed due to interference from the standard broadcast band which is out of the design bandwidth of the particular VLF version of the present invention which was tested. It is reasonably anticipated that passive filtering in the active circuit could effectively reduce these phenomena to negligible levels.

In the VLF version of the present invention as described in connection with FIG. I, the subminiature active antenna of the present invention does not perform any preselection, therefore it does not exhibit the information band limiting properties present in some probe type VLF reception devices which are presently in use. It is anticipated, however, that some form of preselection in either an active or passive form may be desirable if the antenna of the present invention were to be put to use in extremely high level ambient wave fields that may be found on board Navy vessels, for example. Since the preselected bandwidth may be designed to any reasonable information bandwidth and skirt selectivity, no information bandwidth problems should exist because of such preselection.

FIG. 2 illustrates a variant form of the present invention which comprises two substantially circular nonmagnetic conductive plane members 30 and 31 which are connected by a conductor member 32 that may be in the form of a monopole. The conductor 32 passes through an aperture 33 in the plane member 31 and is connected to an active circuit contained within a shielded enclosure 34. The shielded enclosure 34, as well as the active circuit contained therein which may besubstantially of the general type described in connection with the embodiment of FIG. 1, is grounded to the plane member 31. An output terminal 35 provides connection for a coaxial cable to an appropriate RF receiver.

FIG. 3 is a perspective illustration of one actual embodiment of the present invention which performed in a highly satisfactory manner in the VLF frequency range. In FIG. 3 an upper non-magnetic conductive plane member consists ofa sheet of copper approximately 12 inches by 12 inches in size; similarly, a lower non-magnetic conductive plane member 11 is of the same material and size, being spaced in a parallel relationship immediately below the upper member by a distance of approximately 12 inches.

The two plane members, 10 and 11, are maintained in this spaced parallel disposition by sheets of non-conductive, nonmagnetic plastic material 40, 41, and 42. Appropriate reinforcing is employed about the plastic members 40, 41, and 42 at the corner edges as shown at 40A, 41A, and 42A. In the particular embodiment illustrated in FIG. 3, the plastic members 40, 41, and 42 were employed to maintain the antenna members 10 and II in parallel-spaced disposition and a desirable amount of rigidity and structural strength was provided by the reinforcement as shown at 40A, 41A, and 42A. However, it will be appreciated by those skilled in the art that many appropriate arrangements may be devised and employed within the teaching and concept of the present invention to maintain the antenna plane members such as those illustrated at 10 and 11 in the desired parallel-spaced disposition. Different applications of the antenna may impose varying requirements,

but the concept of the teaching of the present invention can be readily adapted to a wide variety of such requirements.

In the illustration of FIG. 3, non-magnetic conductive plane members 10 and 11 comprise sheets of copper. However, such plane members may include any acceptable non-magnetic, conductive plane surface such as fiberglass, bakelite, vinyl, or other nonconductive plastic material supporting a conductive plane surface comprised of either sprayed or painted conductive paint. One of many possible alternatives may comprise conductive plane surfaces plated directly on an appropriate non-conductive framework.

Accordingly, the antenna of the present invention may be constructed by a wide variety of currently available techniques and is ideally adaptable to being permanently sealed as a ruggedized unit. As shown in FIG. 3, the particular embodiment illustrated could readily be sealed within an enclosure of appropriate radome type material so as to include the active circuit 15, completely sealing the antenna assembly against severe environmental conditions.

It should be particularly noted that the active portion of the circuitry, as included in the shielded enclosure 15, need not be mounted between the two non-magnetic conductive plane members 10 and 11 as had been suggested by some prior art concepts. It is desirable, however, that the active portion of the antenna assembly, as represented by the shielded enclosure 15, be mounted in reasonably close proximity to the ground plane member 11 approximately as illustrated in FIG. 3.

It will be readily apparent that the concept of the present invention is not limited to those explicit configurations illustrated in FIGS. 1, 2, and 3 of the drawings, nor is the invention limited to embodiment in the sizes and dimensions given as examples in connection with the description of the operation of the embodiments as illustrated in FIGS. 1, 2, and 3.

Moreover, because of the concept of the present invention, the capacitance multiplier circuit which is the active portion of the antenna assembly, is readily adapted to being embodied in a solid state form such as a fully transistorized circuit, for example.

In summary, the more important advantages of an tennae fabricated in accordance with the teaching and concept of the present invention may be listed as follows:

l. The antenna of the present invention is broadband and therefore does not limit the information bandwidth of the received signal.

' 2. Because the active circuit characteristics are matched to the antenna element characteristics, optimum efficiency is achieved and intermodulation as well as mixing are minimized at tolerable levels.

3. The entire antenna assembly is of relatively extremely small physical size as compared to prior art antennas operating in comparable frequency ranges.

4. The antenna of the present invention, when embodied in a VLF version, is small enough to be installed readily in aircraft and small vessels.

5. The antenna of the present invention requires no additional ground plane in that it includes a self-contained ground plane.

6. The antenna of the present invention needs no tuning.

7. The antenna of the present invention may be constructed readily in a ruggedized form suitable for use in extreme environmental conditions.

8. The antenna of the present invention may be used to replace conventional receiver antenna systems thus significantly reducing the number of components customarily associated with prior art antenna systems.

9. The antenna of the present invention, because of its concept and teaching, lends itself readily to construction and fabrication with a wide variety of materials.

l0. The antenna of the present invention because of its unique and highly desirable configuration and size may be readily fabricated as a permanently sealed unit.

Since many changes can be made in the specific I combinations of apparatus disclosed herein and many apparently different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the foregoing description or shown in the accompanying drawings be interpreted as being illustrative and not in a limiting sense.

What is claimed is:

1. A subminiature active antenna assembly comprising:

a first non-magnetic conductive plane member;

a second non-magnetic conductive plane member of substantially the same dimensions as said first nonmagnetic conductive plane member separated in spaced, parallel relation from said first plane member by a distance of the order of the principal plane dimension of said members;

a conductive member electrically connected to said first plane member substantially at its center and extending therefrom at right angular disposition, through an aperture in said second plane member;

a separate capacitance multiplier circuit positioned beneath the antenna structure comprising said members, and having a ground connection to said second plane member, and a signal input connection to said conductive member for multiplying the passive input capacitance of said members to provide a substantially increased apparent capacitive value to a received electromagnetic wave field; and

said capacitance multiplier circuit having a discrete input impedance of phase and amplitude substantially matching the conjugate discrete passive output impedance presented to it by said antenna structure comprising said members.

2. A subminiature active antenna assembly as claimed in claim 1 wherein said first and second plane members are substantially square and are spaced from each other by a distance substantially equal to the linear edge dimension of the plane members.

3. A subminiature active antenna assembly as claimed in claim 1 wherein said plane members are substantially circular. I

4. A subminiature active antenna assembly as claimed in claim 1 wherein said conductive member is a monopole.

5. A subminiature active antenna assembly as claimed in claim I and operative in the VLF range wherein said plane members are substantially twelve inches square and are separated by substantially twelve inches.

6. A subminiature active antenna assembly as claimed in claim 1 and operative in the VLF range wherein said plane members are circular, having a diameter of the order of twelve inches, and are separated by a distance substantially of twelve inches.

7. A subminiature active antenna assembly as claimed in claim 1 wherein said capacitance multiplier circuit is a solid state transistorized circuit mounted immediately below said second plane member. 

1. A subminiature active antenna assembly comprising: a first non-magnetic conductive plane member; a second non-magnetic conductive plane member of substantially the same dimensions as said first non-magnetic conductive plane member separated in spaced, parallel relation from said first plane member by a distance of the order of the principal plane dimension of said members; a conductive member electrically connected to said first plane member substantially at its center and extending therefrom at right angular disposition, through an aperture in said second plane member; a separate capacitance multiplier circuit positioned beneath the antenna structure comprising said members, and having a ground connection to said second plane member, and a signal input connection to said conductive member for multiplying the passive input capacitance of said members to provide a substantially increased apparent capacitive value to a received electromagnetic wave field; and said capacitance multiplier circuit having a discrete input impedance of phase and amplitude substantially matching the conjugate discrete passive output impedance presented to it by said antenna structure comprising said members.
 1. A subminiature active antenna assembly comprising: a first non-magnetic conductive plane member; a second non-magnetic conductive plane member of substantially the same dimensions as said first non-magnetic conductive plane member separated in spaced, parallel relation from said first plane member by a distance of the order of the principal plane dimension of said members; a conductive member electrically connected to said first plane member substantially at its center and extending therefrom at right angular disposition, through an aperture in said second plane member; a separate capacitance multiplier circuit positioned beneath the antenna structure comprising said members, and having a ground connection to said second plane member, and a signal input connection to said conductive member for multiplying the passive input capacitance of said members to provide a substantially increased apparent capacitive value to a received electromagnetic wave field; and said capacitance multiplier circuit having a discrete input impedance of phase and amplitude substantially matching the conjugate discrete passive output impedance presented to it by said antenna structure comprising said members.
 2. A subminiature active antenna assembly as claimed in claim 1 wherein said first and second plane members are substantially square and are spaced from each other by a distance substantially equal to the linear edge dimension of the plane members.
 3. A subminiature active antenna assembly as claimed in claim 1 wherein said plane members are substantially circular.
 4. A subminiature active antenna assembly as claimed in claim 1 wherein said conductive member is a monopole.
 5. A subminiature active antenna assembly as claimed in claim 1 and operative in the VLF range wherein said plane members are substantially twelve inches square and are separated by substantially twelve inches.
 6. A subminiature active antenna assembly as claimed in claim 1 and operative in the VLF range wherein said plane members are circular, having a diameter of the order of twelve inches, and are separated by a distance substantially of twelve inches. 